Quenching method and apparatus for tempering a glass sheet

ABSTRACT

A quenching method and apparatus for tempering a glass sheet. The method includes a step of transferring a heated glass sheet between quenching boxes which are provided with nozzles opposingly arranged near upper and lower surfaces of the glass sheet to blow air streams of cooling air supplied from the quenching boxes to the glass sheet. The method also includes the step of arranging the nozzles extending from the quenching boxes so as to face at least one of the upper and lower surfaces of the glass sheet so that openings on the end of the nozzles blow the air streams of the cooling air in different directions simultaneously so as to intersect and thus be substantially uniformly arranged with respect to the glass sheet. The number of intersections is approximately 30 or more in any 10 cm square area of the glass sheet. The end portions of the nozzles have a semi-spherical shape. The apparatus includes quenching boxes arranged opposing the surfaces of the glass sheet, and nozzles extend from the quenching boxes so that air streams of cooling air are blown towards the glass sheet which has been heated to a predetermined temperature. Each of the nozzles has a tubular form with a semi-spherically shaped end portion opposing the surfaces of the glass sheet. The end portions of the nozzles each have a plurality of openings so that air streams of cooling air supplied from the quenching boxes are blown towards the surfaces of the glass sheet. The openings formed are uniformly arranged in the end portion of the nozzles.

This application is a Division of application Ser. No. 09/096,179 Filedon Jun. 12,1998 now U.S. Pat. No. 6,180,237.

BACKGROUND OF THE INVENTION

The present invention generally relates to a tempered glass and a methodand an apparatus for quenching a glass sheet to obtain the temperedglass sheet and more particularly, the present invention relates to athin tempered glass having a large surface area and a complicated curvedsurface, such as a back light glass for automobiles, and a quenchingmethod and a quenching apparatus for tempering the glass sheet ofreduced thickness.

DISCUSSION OF BACKGROUND

A tempered glass is used for a window glass for automobiles except for afront windshield glass. There are official regulations on fragmentationof tempered glass from the standpoint of safety so that the driver or apassenger is prevented from injuring. A window glass for automobiles isnot permissible to use unless the window glass satisfies requirementsdescribed in the official regulations.

For example, in one of the regulations on the tempered glass forautomobile windows, there is a regulation concerning a state offragmentation of glass produced when a localized impact is given to thetempered glass. Specifically, an area in which a number of the fragmentsof a glass sheet broken by an impact is minimum and an area in which anumber of the fragments is maximum are selected, and the minimum andmaximum numbers of the fragments in these areas have to fall inpermissible ranges. The maximum size of glass particles produced from afractured glass sheet is determined from a minimum permissible number ofthe fragments. When the maximum size is small, a danger of sufferinginjury from larger fragments is reduced. Further, the minimum size ofthe fragments produced by the fracture of the glass sheet is determinedby a maximum permissible number of the fragments. When the minimum sizeis large, a danger of entering of glass particles into a human body isreduced. ECE standards or JIS standards rule the magnitude and so on offragmentation of glass sheet when fractured. In ECE standards (E6), forexample, it is required that a number of fragments in any 5 cm×5 cmsquare should be 40 at the minimum and 400 at the maximum (except for abelt-like region of 20 mm from the edge of the glass sheet and acircular region of 75 mm radius having the center which is the point ofinitiating breakage). In the following description, the maximum value ofa number of fragments of glass is referred to as the maximum number andthe minimum value is referred to as the minimum number. Further, thereare requirements that when a glass sheet is broken, edges of fragmentsshould not be sharp and elongated fragments having a length of 75 mm ormore should not be produced. Further, there is a requirement that thesurface area of a fragment should not exceed 3 cm².

A tempered glass can be formed by heating a glass sheet to a temperaturenear the softening point of the glass (usually about 600-700° C.) andquenching it by supplying cooling air. The cooling air is blown to theglass sheet through a plurality of cooling nozzles disposed near bothsurfaces of the glass sheet. Thus, a temperature difference is given tothe glass sheet between a surface portion and the inner portion of theglass sheet at the time of quenching so as to from a compressive stresslayer in the glass surface finally solidified, whereby the glass sheetis tempered.

Recently, weight reduction is required for automobiles to reduce fuelcost and so on. With this, there is an increased demand of reducing theweight by reducing the thickness of glass sheets. Using a glass sheet ofabout 4-6 mm thick, a tempered glass satisfying the above-mentionedrequirements can easily be obtained by the above-mentioned glasstempering method (quenching method). However, when a thin glass sheet isto be formed to meet the requirement of weight reduction, it wasdifficult to obtain a tempered glass satisfying the regulations by theabove-mentioned tempering method because a sufficient temperaturedifference could not be form between the surface and the inner portionof the glass sheet due to the glass sheet being thin.

In concepts, there are considered various measures to increase apressure of cooling air; to bring the nozzles closer to the glass sheet;to reduce the distance (pitch) between nozzles and so on in order toprovide a sufficient temperature difference between the surface and theinner portion of the glass sheet. An attempt of increasing a pressure ofcooling air is not realistic because there is a limit in terms ofmechanism in a blowing device or a compressor.

It is necessary that the cooling air is supplied to the glass sheet toassure a way of escape of the cooling air after it impinges on the glasssheet. If the cooling air, after impingement, stays there, the coolingair prohibits successively supplied cooling air from impinging on theglass sheet whereby it is difficult to obtain uniform blowing of coolingair to the glass sheet. When the nozzle pitch is reduced or the nozzlesare brought closer to the glass sheet, the way of escape of cooling air,after the impingement on the glass sheet, can not be assured.

Further, there has been proposed a method of oscillating the glass sheetat the time of blowing cooling air for tempering the glass sheet,whereby the glass surface is uniformly quenched. In this method, whenthe nozzles are brought closer to the glass sheet, the oscillated glasssheet may interfere with the nozzles. In particular, when the glasssheet is shaped to have a complicated curved surface, there is a largepossibility of interfering of the nozzles with the glass sheet.

There has been proposed to conduct a tempering treatment with a specialarrangement of nozzles so that a tempered glass of thin thickness can beobtained. The proposal is to control the propagation of fracture ofglass by forming areas of different principal stress in the glass sheet.

Here, description is made as to a direction of the principal stress anda principal stress difference in a glass sheet. First, a plane which isperpendicular to the glass sheet surface (a cross-sectional plane of theglass plate) is selected from the glass sheet and then, a point isselected from the selected plane. Various angles with a line in parallelto the glass surface are selectable from the selected plane. Stresses ina direction perpendicular to the selected plane acting on this point areunequal depending on angles of the selected plane. So, there is oneselected plane which has the largest stress and the smallest stress,which are perpendicular to each other, when a certain angle is selectedfrom among the various angles. The principal stress direction is definedas the direction of the largest stress and the smallest stress.Hereafter, the direction of the largest stress is referred to as theprincipal stress direction, as representative. Further, the largeststress and the smallest stress (i.e., the stress in the directionperpendicular to the direction which indicates the largest stress) is aprincipal stress difference. In a tempered glass, the principal stressis estimated from the principal stress difference which is obtained witha photoelasticity method. The principal stress difference of thetempered glass corresponds to a value obtained by dividing the sum ofvalues of the difference between the largest stress value and thesmallest stress value at points aligned in the glass sheet thicknessdirection by the thickness of the glass sheet (an average value obtainedby dividing an integrated value of the difference between the largeststress value and the smallest stress value by the thickness). Namely,when a certain point is selected in a surface of the glass sheet, anaveraged integrated value of the difference between the largest stressvalue and the smallest stress value at points aligned in the directionof the thickness from the selected point, is referred to as theprincipal stress difference at the selected point (the principal stressdirection in this case is referred to as the principal stress directionat this point).

For the tempered glass in which there are areas having differentprincipal stress direction, the following proposal is made. U.S. Pat.No. 4,128,690 describes a tempered glass having a thickness of 2.5-3.5mm. The tempered glass has a central tensile stress of 62 MN/m² at themaximum (a surface compressive stress of 124 MN/m² at the maximum). Thetempered glass has a distribution of areas in which the principalstresses acting in the plane of the glass sheet are unequal. Further,there is described in the US patent that in the areas having differentprincipal stresses in the tempered glass, the maximum value of principalstress difference is in a range of 8-25 MN/m² and the distance betweenthe adjacent areas indicating the maximum value is in a range of 15-30mm.

However, when the glass sheet having a thickness of 3.0 mm or less isactually produced as a tempered glass having the above-mentioneddistribution, the following disadvantage is found. In the fragmentationtest according to E6, the difference between a maximum number and aminimum number becomes large. This shows that either an upper limit ofthe maximum number or a lower limit of the minimum number ruled in E6 isapt to be outside even by a slight change of conditions for forming thetempered glass (e.g., an outside air temperature and so on). Thetempered glass having such distribution tends to produce elongatedfragments of glass. Further, the maximum surface area of the fragmentsis generally apt to exceed 3 cm². It is supposed that such tendenciesare derived from a coarse distribution of the areas.

Japanese Unexamined Patent Publication JP-A-55-104935 describes atempered glass of 2.5-3.5 mm thick. The tempered glass has an averagesurface compressive stress of 850-1350 kg/cm² and areas in which theprincipal stresses acting in the plane of the glass sheet are unequalare formed in a scattered state. In such areas, the maximum value of theprincipal stress difference is in a range of 50-300 kg/cm², and thedistance between adjacent areas indicating the maximum value is in arange of from 5 mm or more to less than 15 mm.

In the above-mentioned publication, there is a statement concerning atempered glass of 2.5 mm thick in Example 5. The distance betweenadjacent areas indicating the maximum value of principal stressdifference is 7.1-9.0 mm. Namely, it is understood that use of a thinglass sheet can obtain a tempered glass capable of meeting the officialrequirements if the above-mentioned distance is reduced. The reduceddistance causes an irregular pattern of cracks in the tempered glasswhen the fragmentation test is carried out. The irregular pattern ofcracks is advantageous in obtaining smaller fragments of glass.

However, in forming the irregular pattern of cracks, the development ofcracks depends on nothing, namely, it is difficult to artificiallycontrol the production of cracks. On the other hand, a slight change inthe conditions for forming the tempered glass will result a delicatechange of a magnitude of the stresses or a distribution of the stressesin the tempered glass to be produced. In particular, a thin glass sheetis easily influenced by a slight change of the conditions. Accordingly,if the development of cracks can not be well controlled, it is difficultto estimate a magnitude of the stresses or a distribution of thestresses whereby determination of the forming conditions is difficult.

Japanese Unexamined Patent Publication JP-A-58-91042 describes atempered glass having a thickness of 2.4-3.5 mm in which belt-likeregions having a higher surface compressive stress of 1300 kg/cm² orless at the maximum and belt-like regions having a lower surfacecompressive stress of 1020 kg/cm² or more at the minimum value arealternately formed in its surface. The difference between the maximumvalue and the minimum value of the surface compressive stress is 80-220kg/cm². In the belt-like regions having a lower surface compressivestress, the maximum value of the principal stress difference is 80kg/cm² or more.

Of several kinds of tempered glass described in the publication,attention is paid to a glass sheet of 2.4 mm thick. For such glasssheet, it is required either to reduce the width of the belt-likeregions having a lower surface compressive stress or to increase theprincipal stress value of the belt-like regions having a lower surfacecompressive stress. When the width of the belt-like regions having alower surface compressive stress is reduced, the irregular pattern ofcracks can easily be produced. When the principal stress value of thebelt-like regions having a lower compressive stress is increased, apractically required strength of the tempered glass may not be obtainedbecause the principal stress difference is originally a tensile stress.

With respect to the practically required strength, the followingdisadvantage is, in particular, thought. Namely, the belt-like regionshaving a lower surface compressive stress correspond to portions towhich cooling air streams are not applied during the tempered treatment.Accordingly, for the glass sheet having a thickness of 2.4 mm, it ispractically difficult to render the surface compressive stress of glasssheet portions to which cooling air streams is not applied, to be 1020kg/cm² or more. Accordingly, it is estimated that the surfacecompressive stress in these portions is actually about 900 kg/cm² at themost. The obtained value is insufficient in terms of a practicallyrequired strength in the tempered glass to be produced.

In an example of the above-mentioned publication, there is described anaverage surface compressive stress concerning a glass sheet having athickness of 2.4 mm, i.g., there is described an example of a temperedglass having an average surface compressive stress of 1100 kg/cm² and adifference 190 kg/cm² between the higher and lower surface compressivestresses. The values disclosed therein is an average value between ahigher compressive stress and a lower compressive stress. Accordingly,the publication does not describe a tempered glass in which the surfacecompressive stress in the belt-like regions is, in fact, 1020 kg/cm².

As described above, the tempered glass having a thickness of about 2.5mm in which a distribution of principal stress is formed in the glasssheet is known. However, it was in fact difficult for the tempered glassto satisfy the official requirements when a glass sheet having athickness of about 2.5 mm was used. Further, there were many problemsfor equipment in order to obtain the thin tempered glass meeting theofficial requirements. In particular, there were problems of equipmentfor a glass sheet having a complicated curved shape or a glass sheethaving a large surface area. In concept, the official requirements willbe satisfied by making a distribution of areas in which the principalstresses are different to be dense. For the satisfaction, it isnecessary to use measurements such as bringing the nozzles for supplyingcooling air closer to the glass sheet, reducing the distance (pitch)between adjacent nozzles and so on. Such measures result thebefore-mentioned disadvantage that the way of escape of cooling airafter the impinge with the glass sheet can not be assured.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved temperedglass of thin thickness, a method for cooling a glass sheet and anapparatus for cooling the glass sheet by which the tempered glass caneasily be obtained.

In accordance with the present invention, there is provided a temperedglass comprising a glass sheet having a thickness of 2.3-3.5 mm in whichan average surface compressive stress of 1000-1300 kg/cm² is formed, thetempered glass being characterized in that:

there are formed a plurality of mutually parallel belt-like regions Ahaving a width of 10-30 mm and a plurality of belt-like regions B eachbeing interposed between adjacent belt-like regions A in the glasssheet;

in the belt-like regions A, there are a plurality of reference points ahaving a principal stress difference of 120 kg/cm² or less, which islarger than principal stresses at peripheral areas of the referencepoints a, wherein directions of principal stresses are mutually insubstantially parallel; there exists no point having a larger principalstress difference than principal stress differences between adjacentreference points a; and the shortest lines connecting adjacent referencepoints a form the center line, as the reference line, of each of thebelt-like regions A; and

in the belt-like regions B, there are a plurality of reference points bwhich have a larger principal stress difference than principal stressdifferences at any peripheral areas of the reference points b, and thedirections of principal stress at the reference points b are differentfrom the directions of principal stress at areas adjacent to thereference points b.

There is provided a tempered glass comprising a glass sheet having athickness of 2.3-3.5 mm in which an average surface compressive stressof 1000-1300 kg/cm² is formed, the tempered glass being characterized inthat:

there are formed a plurality of mutually parallel belt-like regions Ahaving a width of 10-30 mm and a plurality of belt-like regions B eachbeing interposed between adjacent belt-like regions A in the glasssheet;

in the belt-like regions A, there are a plurality of reference points ahaving a principal stress difference of 120 kg/cm² or less, which islarger than principal stresses at peripheral areas of the referencepoints a, wherein directions of principal stresses at the referencepoints a are mutually in substantially parallel; there exists no point,between adjacent reference points a, which has a larger principal stressdifference than principal stress differences at any peripheral areas andwhich has a different stress direction from the principal stressdirections at the reference points a, and the shortest lines connectingadjacent reference points a form the center line, as the reference line,of each of the belt-like regions A; and

in the belt-like regions B, there are a plurality of reference points bwhich have a larger principal stress difference than principal stressdifferences at any peripheral areas of the reference points b, and thedirections of principal stress at the reference points b are differentfrom the directions of principal stress at areas adjacent to thereference points b.

In accordance with the present invention, there is provided a quenchingmethod for tempering a glass sheet comprising transferring a heatedglass sheet between a pair of quenching boxes each provided with aplurality of nozzles which are opposingly arranged near both surfaces ofthe glass sheet and which blow to the glass surfaces cooling airsupplied from the quenching boxes, the quenching method beingcharacterized in that nozzles arranged facing at least a side of theglass surfaces are provided with a plurality of openings capable ofblowing the cooling air in different directions simultaneously whereinthe cooling air is blown to the glass surface so that intersections ofblowing directions of air streams of cooling air through the nozzles tothe glass surface are arranged substantially uniform on the glasssurface.

In accordance with the present invention, there is provided a quenchingmethod for tempering a glass sheet comprising transferring a heatedglass sheet between a pair of quenching boxes each provided with aplurality of nozzles which are opposingly arranged near both surfaces ofthe glass sheet and which blow to the glass surfaces cooling airsupplied from the quenching boxes, the quenching method beingcharacterized in that a plurality of nozzles in a tubular form arearranged at at least a side of the glass surfaces wherein each endportion of the nozzles opposing the glass sheet has a convex, curvedshape, and a plurality of openings are formed in the end portion so thatcooling air supplied from the quenching boxes is blown to the glasssheet through the nozzles.

Further in accordance with the present invention, there is provided aquenching apparatus for a glass sheet comprising at least quenchingboxes arranged opposing to both surfaces of the glass sheet and aplurality of nozzles attached to the quenching boxes so that cooling airis blown through the nozzles to the glass sheet heated to apredetermined temperature, the quenching apparatus being characterizedin that each of the nozzles is in a tubular form and has an end portionin a convex, curved shape at a side opposing the glass sheet, and aplurality of openings are formed in the end portion so that cooling airsupplied from the quenching boxes is blown to the glass sheet.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

In drawing:

FIG. 1 is a diagram showing an embodiment of the tempered glassaccording to the present invention;

FIG. 2 is a diagram showing in an enlarged scale a portion of FIG. 1;

FIG. 3 is a vertically cross-sectional view showing an embodiment of thequenching apparatus for a glass sheet according to the presentinvention;

FIG. 4a is a perspective view showing an embodiment of a nozzle used forthe quenching apparatus of the present invention;

FIG. 4b is a plan view from an upper part of the nozzle;

FIG. 5 is a plan view viewed from an upper part of another embodiment ofthe nozzle;

FIG. 6 is a diagram showing an example of a state of blowing cooling airin the present invention;

FIG. 7 is a diagram showing a measuring device used for obtaining aprincipal stress direction and a principal stress difference; and

FIGS. 8a and 8 b are respectively a front view and a cross-sectionalview for explaining dimensions of the glass sheet.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described indetail with reference to the drawings.

FIG. 1 is a diagram showing an embodiment of the tempered glassaccording to the present invention and FIG. 2 is a diagram showing in anenlarged scale the tempered glass, which are for explaining principalstresses formed in the tempered glass. Arrow marks in the Figures showthe principal stress direction at each reference point. Larger circlesindicate points to which cooling air streams impinge the glass sheet andsmaller black points indicate reference points.

The thickness of a tempered glass 10 to be treated is in a range of2.3-3.5 mm. In a special case wherein conditions such as outside airtemperature and so on for quenching the glass sheet are not met, theperformance of the cooling air, which described below, is sometimesinsufficient. Accordingly, the thickness of the glass sheet which ismore realistic and is preferable to obtain efficiently the temperedglass of the present invention is 2.5-3.1 mm. The compressive stress inaverage which is formed in the surface of the tempered glass is in arange of 1000-1300 kg/cm². Belt-like regions A and belt-like region Bare alternately formed in the surface of the tempered glass 10.

Each of the belt-like regions A is a belt-like region having a width Lain a range of 10-30 mm in which the reference line T as the center lineis formed by a group of reference points a. The reference points a arepoints having a principal stress difference of 120 kg/cm² or less, whichis larger than principal stress differences at any point around thereference points a. In addition, the reference points a are pointsselected so as to satisfy the following conditions: (a) there is nopoint having a larger principal stress difference than that at any otherpoint between adjacent reference points a, or (b) there is no pointhaving a principal stress direction which is different from theprincipal stress direction of the reference points a and having a largerprincipal stress difference than the principal stress difference at anypoint in the vicinity of the reference points a. The principal stressdirections at the reference points a are mutually in substantiallyparallel and are substantially perpendicular to the longitudinaldirection of the belt-like regions A. The sum (Lc+Ld) of the distance Lcbetween a pair of adjacent reference points a, a and the distance Ldbetween the another pair of adjacent reference points a, a is in a rangeof 40-60 mm.

In each of the belt-like regions B, there are dispersively a pluralityof reference points k each having a larger principal stress differencethan a principal stress difference at a point in the vicinity of thereference points b. A principal stress direction at a reference point bis different from a principal stress direction at another referencepoint b which is adjacent thereto.

In the tempered glass 10 shown in FIGS. 1 and 2, a line formed bytracing the principal stress directions of reference points b in abelt-like region B forms a snaking line. A principal stress direction atreference points b in the vicinity of the border line of an adjacentbelt-like regions A (hereinbelow, referred to as reference points b_(A))is substantially perpendicular to the principal stress direction of thereference points a.

In the tempered glass 10 of this embodiment, the reference points b areclassified into first and second groups. Namely, the reference points bare arranged as follows The reference points b belonging to the firstgroup have their principal stress directions which are substantiallyperpendicular to the principal stress directions of the reference pointsa. Further, they are alternately located in the vicinity of the borderof either an adjacent belt-like region A or the other adjacent belt-likeregion A opposing thereto with respect to the reference line T. On theother hand, the reference points b belonging to the second group havetheir principal directions which substantially coincide with directionsformed by connecting reference points b in the vicinity of the border ofan adjacent belt-like region A in the first group and reference points bin the vicinity of the border of the other adjacent belt-like region Aopposing thereto, the reference points b of the second group beinginterposed between adjacent reference points b of the first group.

Description has been made so that the principal stress directions at thereference points a are substantially parallel. Further, description hasbeen made so that the principal stress directions at the referencepoints b_(A) are substantially perpendicular to the principal stressdirections at the reference points a. However, it should be understoodthat the terms “substantially parallel” or “substantially perpendicular”means that it is not always necessary to be strictly “Parallel” or“perpendicular” in the present invention. Namely, the above-mentionedprincipal stress directions should be “parallel” or “perpendicular” toan extend that the thin glass sheet has a distribution of stress capableof satisfying the official regulations. More detailed description willbe made of this. The tempered glass of the present invention is formedby blowing cooling air for quenching to a heated glass sheet. In thiscase, a distribution of stress in the glass sheet surface can beobtained by desirably scattering points of impingement of cooling airstreams on the glass sheet surface. On the other hand, the formation ofthe principal stress difference relys on a degree of quenching. However,it is difficult to keep the degree of quenching to be constant due tovariations of outer air temperature and so on. Therefore, even thoughconditions for blowing the cooling air are determined so that theprincipal stress directions are oriented to predetermined directions(parallel or perpendicular directions), predetermined stress directionsare not always oriented correct directions (parallel or perpendiculardirections). In consideration of the above-mentioned circumstances, thetempered glass of the present invention includes such tempered glassthat can satisfy the official regulations even though the principalstress directions are slightly different from desired directions(parallel or perpendicular directions). However, it is preferable forthe principal stress directions to orient strictly predetermineddirections (parallel or perpendicular directions) from the viewpoint ofcontrolling accurately the development of cracks.

The reason why the tempered glass having such distribution of principalstresses can satisfactorily meet the official requirements even thoughthe thin glass sheet, is as follows. First, the tempered glass of thepresent invention has the following characteristics:

(1) 120≦σ_(a) preferably, 40≦σ_(a)≦80 (unit:kg/cm², σ_(a): the principalstress difference of the reference points a, and

(2) 10≦L_(a)≦30 (unit: mm)

Further, it is preferable to provide the following characteristics:

(3) 10≦L_(b)≦30 (unit: mm)

(4) 50≦σ_(b) (unit: kg/cm², σ_(b):the principal stress difference of thereference points b)

(5) a line formed by tracing the principal stress directions at thereference points b is snaked, and the principal directions at thereference points b_(A) are substantially perpendicular to the principalstress directions at the reference points a, and

(6) 40≦L_(c)+L_(d)≦60, 20≦L_(c), L_(d)≦30, preferably, L_(c)>L_(d). Thegroup of reference points b in the vicinity of the reference pointsb_(A) correspond to L_(c) and the adjacent thereto corresponds to L_(d).

The number and the magnitude of fragments of the glass sheet in thefragmentation test depends on the behavior of the development of cracks.Namely, when cracks develop linearly, fragments tend to be elongatedpieces and to be larger. Accordingly, it is required not to develop thecracks linearly.

Basically, the linearly developing of cracks can be prevented by thepresence of the principal stresses because cracks tend to bend in thedirection perpendicular to the principal stress directions. When a crackturns and joins another crack, a fragment is formed. In the temperedglass of the present invention, since reference points b havingdifferent principal stress directions are scattered in the belt-likeregions B, cracks run from the belt-like regions A to the belt-likeregions B. However, when a value of σ_(a) is excessively large, cracksdeveloping in the belt-like regions A can not be directed to thebelt-like regions B even though the reference points b exist in thebelt-like regions B.

Accordingly, when the above-mentioned condition (1) is satisfied, cracksdeveloping in parallel to the longitudinal direction in the belt-likeregions A are prevented from being continuously developing in thelongitudinal direction (the principal stress directions at the referencepoints a are to lead the cracks in the direction in agreement with thedeveloping direction of the cracks). When 40>σ_(a), it is difficult tocontrol the development of cracks. On the other hand, when 80<σ_(a),there may occur a delicate restriction of the principal stressdifference or the principal stress direction of the reference points b.Accordingly, 40≦σ_(a)≦80 is in particular preferable.

There is a case of difficulty in preventing the development of crackseven though the condition (1) is satisfied. It is the case that L_(a) isexcessively large. Namely, the role of bending the direction ofdeveloping cracks running in the belt-like regions A is born by theprincipal stresses at the reference points b. Then, there is providedL_(a)≦30 so that cracks are influenced by the principal stresses at thereference points b. On the other hand, when L_(a) is too small, it isdifficult to bend the cracks running from the belt-like regions B to thebelt-like regions A. Accordingly, both conditions (1) and (2) should besatisfied.

The reason why the satisfaction of the condition (3) is preferred issubstantially the same as the reason that the condition (2) should besatisfied. On the other hand, in the tempered glass which satisfies thecondition (5), cracks running in the belt-like regions B are bent in thedirection perpendicular to the principal stress directions at thereference points b. Accordingly, cracks of snaking form can be formedand a desired development of cracks is obtainable even when there isslightly outside the range of (3).

On the other hand, in order to lead the development of cracks in adesired direction, the tempered glass should satisfy the condition (4).When σ_(b)<50, it is difficult to control the development of cracks.

When the principal stress directions at the reference points b_(A) aresubstantially perpendicular to the principal stress directions at thereference points a, cracks running in the belt-like regions A can easilybe bent toward the belt-like regions B whereby the development of thecracks in the longitudinal direction of the belt-like regions A can beprevented. The reason why L_(c)>L_(d) is in particular preferable in thesatisfaction of the condition (6) is as follows. When a distance betweenadjacent reference points a is larger, larger fragments are apt to beproduced at this area. Accordingly, it is required to bend cracksdeveloped in the belt-like regions A toward the belt-like regions B tothereby control the production of larger fragments. Therefore, it isdesirable to provide reference points b_(A) in the vicinity of adjacentreference points a, the distance of which is relatively large.Accordingly, in the determination of L_(c)>L_(d), cracks developing inthe belt-like regions A can be bent toward the belt-like regions Bwhereby the production of larger fragments in the belt-like regions Acan be prevented.

As described above, several factors matually influence in controllingthe cracks. In particular, in use of a thin glass sheet, it is difficultto form a temperature difference between the surface and the innerportion of the glass sheet. Accordingly, the fragmentation meeting theofficial requirements can not be obtained by the conventional method forcontrolling the development of cracks. According to the presentinvention, the tempered glass which satisfies the conditions (1) and (2)is proposed. In addition, it is preferable for the tempered glass tohave the principal stress characteristics of the conditions (3) to (6).

In the following, description will be made as to a method of measuringthe stress values.

(A) Measurement of a Surface Compressive Stress

JIS R3222 is applicable to the measurement of surface compressivestresses. JIS R3222 concerns heat strengthened glass. In themeasurement, the tempered glass of the present invention is measured assamples. The above-mentioned Japanese standards describes points to bemeasured. However, in the measurement of compressive stresses in thetempered glass of the present invention, a plurality of optionallyselected points are measured irrespective of the standard. Then, anaverage value of surface compressive stress is obtained from themeasured values at the plurality of points.

For the measurement of the points, it is preferable to select points ina circular area of 75 mm radius around the central point of the glasssheet. In particular, it is preferable to select respectively the samenumber of points in which the surface compressive stress value isexpected to have the maximum value and the surface compressive stressvalue is expected to have the minimum value. Further, the surfacecompressive stress value should be the maximum at a point where theblowing direction of a cooling air stream for cooling the glass sheetintersects, the surface of the glass sheet, and the surface compressivestress value should be the minimum at the middle point between adjacenttwo points of intersection.

(B) The Measurement of a Principal Stress Direction and a PrincipalStress Difference

FIG. 7 shows a measuring device for measuring a principal stressdirection and a principal stress difference. In the basic structure, aprincipal stress direction and a principal stress difference can beobtained by introducing a circular polarized light into the temperedglass 10 and measuring a polarized state of an elliptically polarizedlight which is formed by a strain of the tempered glass 10 and passingtherethrough. Light emitted from a light source 41 is passed through apolirizer 42 to form a linearly polarized light. Then, the linearlypolarized light is passed through a ¼ λ retardation plate 43 to form acircularly polarized light. An analyzer 45 is disposed behind thetempered glass 10.

The tempered glass 10 is placed perpendicularly to the incident light.The circularly polarized light incident into the tempered glass 10transmits through the tempered glass 10 to be an elliptically polarizedlight which is depending on a stress-strain of the tempered glass 10.The thus obtained elliptically polarized light is introduced into theanalyzer which is rotated. The light passing through the rotatinganalyzer 45 is introduced into a light detection element 46. Bymeasuring an output of the light detecting element 46, a state of theelliptically polarized light can be detected.

A principal stress direction and a principal stress difference can beobtained as follows from a state of the obtained elliptically polarizedlight. When θ₁ and θ₂ represent principal stress directions and δrepresents a phase difference corresponding to a principal stressdifference expressed by the Formula (5) as shown hereafter, an output I(φ) of the light detection element is expressed by the following Formula(1):

 I(φ)=k{1−sin δ·sin 2(θ−φ)}  (1)

where k is a proportionality factor and φ is an angle of rotation of theanalyzer. The ratio of the minimum value I_(min) to the maximum valueI_(max) of an output on the analyzer represents an ellipticity R. R andδ are linked in the following Formula (2):

R=I_(min)/I_(max)=(1−sin δ)/(1+sin δ)  (2)

(where δ>0)

Accordingly, the phase difference δ and principal stress directions θ₁,δ₂ are expressed by the following Formulas (2)′, (3) and (4):

δ=sin⁻¹{(1−R)/(1+R)}  (2)′

θ₁=φ+π/4±nπ  (3)

θ₂=φ−π/4±nπ  (4)

A principal stress difference Δσ is expressed by the following Formula(5). $\begin{matrix}{{\Delta \quad \sigma} = {{\lambda/\left( {c \cdot t} \right)} \times \frac{\delta}{360}}} & (5)\end{matrix}$

where λ is the wavelength of the light emitted from a light source 41(in this measuring device, λ=632.8 nm), c is the photoelasticityconstant (c=2.63 nm/cm/kg/cm² , and t is a thickness of the temperedglass 10).

Namely, the principal stress difference and the principal stressdirection can be obtained by obtaining an ellipticity R of theelliptically polarized light and an angle of rotation φ of the analyzer(an angle of a long axis of an ellipse when the maximum and minimumoutput values are obtainable).

In the stress measuring device, a He—Ne laser is used for the lightsource 41 because laser beams can be throttled to a small point so thata slight change of the tempered glass such as uneven tempering can bedetected. A Gram-Tompson prism having excellent polarizingcharacteristics was used for the polarizer 42. A glass plate 47 wasdisposed between the polarizer 42 and the ¼ λ retardation plate 43 totake the reference light. An interference filter 49 was disposed betweenthe glass plate 47 and a reference light detector 48 so as to minimizethe influence of outer light. The ¼ λ retardation plate 43 used was suchone that was formed by polishing quartz to produce a phase difference ofπ/2 to a wavelength of 632.8 nm. The rotating analyzer 45 used was thesame as the analyzer 42. For the light detecting element 46, a solarcell with an interfering filter at its front side was used so as tominimize the influence of outer light in the same manner as thereference light detector 48.

Thus, the reference points a, b in the present invention can bedetermined by measuring the principal stress differences and principalstress directions at a large number of points. Further, by determiningthe reference points a and b, L_(a), L_(b), L_(c) and L_(d) can bedetermined.

A preferred method for producing the tempered glass of the presentinvention will be described.

FIG. 3 is a diagram in cross section showing an embodiment of thequenching apparatus used for tempering a glass sheet according to thepresent invention. The quenching apparatus comprises mainly quenchingboxes 11, 11′ disposed so as to oppose each surface of a glass sheet 1and a plurality of nozzles 10 attached to the quenching boxes 11, 11′ atpositions facing the glass sheet 1. Outer configurations defined by eachgroup of the nozzles 12 attached to the quenching boxes 11, 11′substantially correspond to the shape of the glass sheet 1 respectively.The glass sheet 1 which has been heated to about a glass softening pointin a heating furnace (not shown) and has been bent depending onrequirement is transferred in a horizontal state between the quenchingboxes 11, 11′ by means of a suitable transferring means such as a ringor the like which is connected to a driving mechanism.

While the glass sheet is between the quenching boxes 11, 11′, coolingair is blown at predetermined temperature and pressure to the glasssheet 1 through each of the nozzles whereby the glass sheet 1 is rapidlycooled to be desirably tempered.

FIGS. 4a and 4 b are respectively a perspective view and a plan viewshowing an embodiment of a nozzle attached to the quenching apparatus ofthe present invention. The nozzle is in a tubular form in which the endportion 12 a facing the glass sheet 1 has a partially spherical shapewhich is protruded toward the glass sheet 1. Namely, the end portion hasa convex, curved surface.

At the end portion 12 a, a plurality of openings 20 are provided tosupply cooling air toward the glass sheet 1. The openings 20 areuniformly provided in the convex, curved surface area of the end portion12 a in, for example, a pattern that they are distributed radiallyaround the center line in the longitudinal direction of the nozzle. FIG.5 is a plan view showing another embodiment of the arrangement of thenozzles according to the present invention.

Thus, use of the nozzle in which the end portion is in a convex, curvedshape and has a plurality of openings, assures the way of escape of thecooling air after it has been blown to the glass sheet, and can increasethe number of air streams per unit surface area on the glass sheetsurface.

As shown in FIG. 6, cooling air is blown to the glass sheet 1 from eachof the nozzles 12 so that points of intersection P of lines of blowingdirection of air emitted from the nozzles 12 to the surface of the glasssheet 1 are arranged substantially uniform on the glass sheet 1. In thiscase, it is preferable that the number of points of intersection P is 30or more per 10 cm square on the glass sheet 1. For this, a nozzle pitch,a distribution of openings formed in the nozzles and a distance betweenthe free end of the nozzles and the glass sheet should properly beadjusted.

In the adjustment, it is preferable that an angle formed by a line ofblowing direction and the surface of the glass sheet is 45° or less, anda distance between the free end of the nozzle and the surface of theglass sheet is about 4-6 times as large as the diameter of the opening,whereby fragments of glass as described in the official requirements canbe obtained as well as providing a sufficient strength of glass sheet.Thus, a distribution of stress in a surface direction of the glass sheetcan be made dense without reducing cooling efficiency (i.e., a distancebetween adjacent areas in which the principal stresses acting in theplan of the glass sheet are different is made short), and fragments atthe breakage of the glass sheet can be small.

In particular, when a thin glass sheet was used and small fragments aredesired to obtain, it was necessary to increase a cooling power ofcooling air. In principle, even though a thin glass sheet having athickness of about 2.3-3.5 mm is used, for instance, small fragments canbe obtained at the breakage by increasing the cooling power of coolingair. However, when the cooling power is increased, the glass sheet maybe broken at the initial stage of cooling because of a tensile stresswhich is temporarily produced in the glass sheet surface. Further, asdescribed before, it is in fact difficult from a view of mechanicalstructure to increase intensively the cooling power. However, thepresent invention makes it possible to provide a glass sheet which canresult small fragments at the breakage without intensively increasingthe cooling power.

Further, when the glass sheet is of a complicated shape (such as a glasssheet which is bent to have a three-dimensionally complicated curvedsurface, in particular, it has a depth of 20 mm in an arched portion ofa complicated curved surface), it is difficult to give a oscillation tothe glass sheet so as to uniformly cool it. In the present invention,however, a stroke of oscillation to be given to the glass sheet betweenthe quenching boxes can be short because the nozzles having theabove-mentioned construction can allow a dense arrangement of theopenings.

The construction of the quenching apparatus for a glass sheet accordingto the present invention is not limited to that as described above. Forinstance, the glass sheet may be held and transferred between thequenching boxes in a horizontal state or a vertical state. In connectionwith this, the quenching boxes may not be disposed in a verticallyopposing relation as shown in the Figure, but may be arranged so as tomeet a state of the glass sheet which is vertically held.

The shape of surface area formed by the group of the nozzles can bedetermined as desired depending on a shape of the glass sheet. However,to the glass sheet, it is preferable from the viewpoint of beingapplicable to the glass sheet with a predetermined stress that adistance between the free end of each of the nozzles to the glass sheetis substantially equal for all of the nozzles. Accordingly, the shape ofsurface area formed by the group of the nozzles should be substantiallyin agreement with a shape of the glass sheet. Further, the shape of asurface area formed by the group of the nozzles can be determined,irrespective of a shape of the glass sheet, by suitably controlling atemperature, an air quantity, a pressure and so on of cooling air fromeach of the nozzles, or suitably adjusting a nozzle pitch.

The quenching boxes may be connected to a blower or a compressor so thatthe cooling air is supplied from the blower or the compressor throughthe quenching boxes and each of the nozzles to the glass sheet. Or thequenching boxes may be divided into a plurality of blocks to whichcooling air is supplied from the blower or the compressor.

The shape, the number and positions of the openings formed in each ofthe nozzles may be identical or not identical although these featuresare determined depending on a distribution of stress to be applied tothe glass sheet.

The shape of the glass sheet is not in particular limited and it may bebend-shaped to have a predetermined radius of curvature or is in a flatform. For the purpose of the present invention, the quenching apparatusis in particular effective for cooling a glass sheet which isbend-shaped to have a complicated curved surface.

The temperature of cooling air which is just before being blown to theglass sheet is preferably at or near a glass softening point such as620-700° C. This is because a glass sheet for automobile is usuallyheated to a temperature capable of bend-shaping, and a temperingtreatment is conducted just after a bend-shaping step.

The glass sheet to which a tempering treatment is conducted to provideeffectively a predetermined stress with use of quenching apparatus ofthe present invention, should have a thickness of 2.3-3.5 mm. In theconventional quenching apparatus, when a thin glass sheet was to betempered, it was difficult to obtain a tempered glass which wasfractured into small fragments at the breakage unless the cooling powerwas increased. On the other hand, in the quenching apparatus of thepresent invention, a tempered glass capable of fracturing into smallfragments at the breakage can be provided without increasing the coolingpower.

For the tempered glass of the present invention, a glass sheet to whichthe quenching method or the quenching apparatus of the present inventionis used, preferably has the dimensions of about (800-1500)×(500-1000)mm.Further, a depth in an arched portion formed in the glass sheet ispreferably in a range of 10-30 mm.

Now, the present invention will be described in detail with reference toexamples. However, it should be understood that the present invention isby no means restricted by such specific examples.

Glass sheets having dimensions as shown in Table 1 were tempered underthe cooling conditions shown in Table 1 to prepare several kinds oftempered glass of Examples 1-10. Further, a simulation of temperingtreatment was conducted to the glass sheets having dimensions shown inTable 1 under the cooling conditions in Table 1 to prepare models oftempered glass of Examples 11-14. In Examples 1-5, the temperingtreatment was conducted by using the apparatus shown in FIGS. 3 and 4.In Examples 6-10, the tempering treatment was conducted by using anapparatus having conventional nozzles (arranged with a large nozzlepitch).

In Table 1, “depth” (W) means a depth (unit: mm) of a bent portion (anarched portion) in a short side direction of the glass sheet;“temperature” means a temperature (unit: ° C.) of the glass sheets atthe time of initiating cooling; and “distance” means a distance(unit:mm) from the free end of the nozzles to the glass sheet surface.In addition, mm is used for the unit of a thickness (t) and dimensions(x×y) of the glass sheets, and mmAq is used for the unit of windowpressure of cooling air. In determining the dimensions, FIG. 8 isreferred to. The depth W is determined to have the greatest value inFIG. 8.

The thus obtained tempered glass indicated physical values shown inTable 2. The maximum surface compressive stress σ_(max), the minimumsurface compressive stress σ_(min), the principal stress differencesσ_(a), σ_(b) and the distances L_(a), L_(b), L_(c) and L_(d) in Table 2were obtained by the measurement of the above-mentioned measurements:(A) the measurement of the surface compressive stress and (B) themeasurement of the principal stress direction and the principal stressdifference. The unit for L_(a), L_(b), L_(c) and L_(d) is mm and theunit for σ_(max), σ_(min), σ_(a) and σ_(b) is kg/cm².

The glass sheets to which the tempering treatment was conducted weresubjected to tests similar to the fragmentation tests for tempered glassdescribed in JIS R3212 to obtain a result shown in Table 3 (wherein “−”means unmeasured). Further, in Table 3, “impact point” means an impactpoint to a sample having a complecated curved surface in the test ofaccording to JIS R3206; “N_(min)” (the minimum number) indicates thenumber of fragments in the area having the smallest number of fragmentsamong areas of 50×50 mm square which are positioned around the impactpoint and are 75 mm apart from the impact point (JIS R3211, Table 5which concerns the behavior of fragments of tempered glass as a safetyglass for automobile); “N_(max)” (the maximum number) indicates thenumber of fragments in the area having the largest number of fragmentsin areas of 50×50 mm square which are positioned around the impact pointand are 75 mm apart from the impact point in the above-mentionedbehavior of fragments; and “number of elongated fragments” indicates thenumber of elongated fragments having a length ranging from 75 mm to 150mm; and “surface area of large fragments” indicates the surface area ofthe largest fragment (unit: cm²).

TABLE 1 Exam- Thick- Dimension Depth Wind Temper- Dis- ple ness (x × y)(w) pressure ature tance 1 2.5 1300 × 800 18 2400 650 10 2 2.8 1300 ×800 18 2000 650 10 3 3.2 1300 × 800 18 1400 650 10 4 2.65 1300 × 800 202100 650 10 5 2.65 1300 × 800 19.5 2100 650 10 6 3.2 1300 × 800 18 2300650 50 7 2.5 1300 × 550 20 2300 650 50 8 2.8 1300 × 550 20 2200 650 45 92.5 1200 × 450 18 1600 650 45 10 2.8 1200 × 450 18 1600 650 45 11 2.51300 × 800 18 2000 650 10 12 2.5 1300 × 800 18 2400 650 10 13 2.5 1300 ×800 18 2400 650 10 14 2.5 1300 × 800 18 2400 650 10

TABLE 2 Example σ_(min) σ_(max) σ_(a) σ_(b) L_(a) L_(a) L_(c) L_(d) 11000 1200 60 60 20 30 30 20 2 1000 1250 60 60 20 30 30 20 3 1000 1300 6060 20 30 30 20 4 1000 1250 65 65 20 30 30 20 5 1000 1250 65 65 20 30 3020 6 — — — — — — — — 7 — — — — — — — — 8 — — — — — — — — 9 — — — — — — —— 10 — — — — — — — — 11 1000 1200 60 60 40 10 30 20 12 1000 1200 60 6010 40 30 20 13 1000 1200 60 60 10 30 20 30 14 1000 1200 60 30 10 30 3020

TABLE 3 Number of Surface area Impact elongated of large Example pointN_(min) N_(max) fragments fragments 1 1 102 352 0 — 2 158 308 0 — 3  74395 0 — 4  99 277 0 2.6 2 1 220 330 0 — 2 205 376 0 — 3 172 281 0 — 4177 374 0 — 3 3 130 — 0 — 4 3  75 275 0 2.6 5 3  71 296 0 2.1 6 3 100 —— — 7 3  1 — — — 8 3  54 — — — 9 3  10 — — — 10 3  34 — — — 11 3 — — 1 —12 3 — — 0 3.1 13 3 — — 0 3.2 14 3 — — 0 4.1

In comparison of Example 3 with Example 6, it is found that in order toobtain the tempered glass satisfying the requirements of the safetyglass for automobile, the supply of cooling air under a window pressureof 1400 mmAq is sufficient in Example 3 (the Example using the methodand apparatus of the present invention), whereas the cooling air of awindow pressure of 2300 mmAq is needed in Example 6 (the Example usingthe conventional nozzles).

Further, in Examples 6-10, the minimum numbers are small in comparisonwith Examples 1-5. From the fact, it is found that in the conventionalapparatus, it is difficult to arrange the nozzles to be dense in orderto assure the way of escape of cooling air and it is difficult to obtainsmall fragments.

In comparison of Example 1 with Example 11, it is found that when σ_(a)is small with respect to L_(a) (when L_(a) is imbalancedly larger thanσ_(a)), the development of cracks in the belt-like regions A can not beprevented whereby small fragments are produced. In comparison of Example1 with Example 12, it is found that when L_(b) is imbalancedly largerthan L_(c) or L_(d), the crossing of cracks in the belt-like regions Bcan be obtained whereby large fragments are produced. In comparison ofExample 1 with Example 13, it is found that when L_(c)<L_(d), largefragments are produced unless L_(a) is made small. In comparison ofExample 1 with Example 14, it is found that an excessively small σ_(b)produces large fragments.

In the tempered glass meeting the principal stress characteristics (1)and (2), and preferably, further meeting the principal stresscharacteristics (3)-(6) as described above, the development of crackscan be controlled so that the fragmentation satisfying the officialrequirements can be obtained. Even by using a thin glass sheet, atempered glass meeting the requirements can be obtained.

Further, the present invention is to provide nozzles each having aplurality of openings at its free end portion which has a convex, curvedshape so that cooling air supplied from quenching boxes is blown to aglass sheet. The nozzles of the present invention is applicable to athin glass sheet to which a sufficient strength could not be provided bythe conventional tempering method and apparatus, and the presentinvention can provide an excellent effect to produce a tempered glasshaving a sufficient strength which satisfies requirement of safety of awindow glass for automobile.

In accordance with the present invention, it is obtainable an excellenteffect to realize a sufficient strength in a tempered glass with a smallquantity of air in comparison with the conventional tempering method andapparatus which were applied to a thicker glass sheet.

What is claimed is:
 1. In a quenching method for tempering a glass sheetcomprising transferring a heated glass sheet between a pair of quenchingboxes each provided with a plurality of nozzles which are opposinglyarranged near both surfaces of the glass sheet and which blow to theglass surfaces cooling air supplied from the quenching boxes, thequenching method being characterized in that nozzles arranged facing atleast a side of the glass surfaces are provided with a plurality ofopenings capable of blowing the cooling air in different directionssimultaneously wherein the cooling air is blown to the glass surface sothat intersections of blowing directions of air streams of cooling airthrough the nozzles to the glass surface are arranged substantiallyuniform on the glass surface.
 2. The quenching method according to claim1, wherein the number of the intersections on the glass surface are 30or more in any 10 cm square.
 3. In a quenching method for tempering aglass sheet comprising transferring a heated glass sheet between a pairof quenching boxes each provided with a plurality of nozzles which areopposingly arranged near both surfaces of the glass sheet and which blowto the glass surfaces cooling air supplied from the quenching boxes, thequenching method being characterized in that a plurality of nozzles in atubular form are arranged at at least a side of the glass surfaceswherein each end portion of the nozzles opposing the glass sheet has aconvex, curved shape, and a plurality of openings are formed in the endportion so that cooling air supplied from the quenching boxes is blownto the glass sheet through the nozzles.
 4. In a quenching apparatus fora glass sheet comprising at least quenching boxes arranged opposing toboth surfaces of the glass sheet and a plurality of nozzles attached tothe quenching boxes so that cooling air is blown through the nozzles tothe glass sheet heated to a predetermined temperature, the quenchingapparatus being characterized in that each of the nozzles is in atubular form and has an end portion in a convex, curved shape at a sideopposing the glass sheet, and a plurality of openings are formed in theend portion so that cooling air supplied from the quenching boxes isblown to the glass sheet.
 5. The quenching apparatus according to claim4, wherein the plurality of openings formed in the end portion of thenozzles are uniformly arranged.